This invention relates to chemical resistant polymer coatings, and is particularly directed to the provision of chemical warfare agent resistant polysiloxane coatings on substrates such as structural metals, e.g. aluminum.
Survival of aircraft in a chemical warfare (CW) environment requires that the aircraft materials of construction be able to withstand the effects of either the CW agents and/or the subsequent decontamination procedures required to remove these agents. The problems created by these CW agents are: (1) corrosion of aircraft materials and structures; (2) swelling and degradation of seals, insulation, and transparencies; and (3) deterioration of avionic components and wiring. Additionally, current decontamination agents and procedures are equally severe and can also have a detrimental effect on aircraft structures and components. To change the aircraft materials of construction for protection against a specific agent also becomes impractical. Thus, a coating that can be applied to all areas of an aircraft which will exhibit minimal adsorption and/or absorption of the CW agent and from which the agents can be readily desorbed by washing with soap and water is a particular object of the invention.
Various procedures have been considered for minimizing the effect of CW agents on aircraft materials of construction. The basic process is to allow the contaminant to make contact with the aircraft and then resort to decontamination procedures. However, the decontamination process is quite severe with a serious adverse effect on the aircraft materials of construction.
Another method is to coat the surface with a water-soluble polymer into which the CW agent is absorbed. Subsequently, this coating is washed off with water and the substrate is protected. This process has the limitation of being applicable to surfaces that would not be affected by water, e.g., metal or composite. Sensitive avionic equipment might sustain damage from the water. Additionally, if the outer surface was so protected and the aircraft was flying through rain, or was being washed down, it would remove the coating. Thus, this becomes somewhat impractical.
Another method that has been considered is to coat the surface with parylene. This is a good surface, but impractical to apply on a large area. It would be effective for small parts because the method of application involves heating a monomer (para cyclophane) under vacuum and passing the resultant diradical that forms into a chamber at low temperatures, at which point the parylene polymer will form by condensing on a substrate. This is quite impractical for a large surface structure.
Hence, another object of the invention is to provide a liquid polymer that can be applied to any substrate and which will form a coating with a "low sticking coefficient." By definition, a "low sticking coefficient" coating is one on which little material would be adsorbed and relatively little would be absorbed after long exposure time. Additionally, it would also allow for a relatively rapid desorption, and from which the adsorbed agent can be removed by washing with soap and water. Teflon is a good example of a "low sticking coefficient" material, but teflon cannot be used since it is not possible to make a continuous film of teflon from solution. It can only be applied to substrates by sintering teflon powder at high temperature to make a contiguous coating. Commercial teflon film is not practical since it cannot be made to readily adhere to most substrates.
Polydimethyl siloxane (silicone rubber) has been used as coatings, but such coatings have not proven sufficiently resistant to CW agents or their simulants. Moreover, to cure a conventional siloxane of this type, it is necessary to treat with a peroxide at high temperature, posing problems for preparation of coatings, particularly for large area surfaces, and hence is not feasible.
Mixtures of various siloxane components for reaction to form a polymer are known. Thus, it is known to react unsaturated siloxanes with cross linking agents to produce siloxane polymers. However, application of such mixtures to a substrate for formation of a coating resistant to chemical warfare agents has not heretofore been achieved.
In U.S. application Ser. No. 07/604,877, pending, filed of even date herewith, by Norman R. Byrd, and assigned to the same assignee as the present application, there is disclosed a coating composition in the form of a liquid mixture of (a) a polysiloxane having unsaturated groups available for cross-linking, such as a vinyl group, and (b) a polysiloxane containing reactive Si-H groups. A small amount of a platinum siloxane catalyst is also present. Such liquid coating composition can be applied to a substrate at room temperature and cured at room temperature to form a coating resistant to CW agents or simulants. However, in some instances, the surfaces of the resulting coatings have bubbles due to liberation of hydrogen from the Si-H groups of component (b). Such bubbly coatings form a rough surface which may disadvantageously promote CW agent or simulant absorption.
Accordingly, still other objects of the invention are the provision of improved mixtures of modified siloxanes containing an additive, and which upon curing form smooth bubble-free coatings which show minimal adverse effects after exposure to CW agents or simulants, the application of such mixtures to a substrate such as aluminum, and the resulting cured improved coating systems which are resistant to CW agents or simulants thereof.